1. Field of the Invention
The present invention relates to photoresist polymers and photoresist compositions containing the same, and in particular to photoresist polymers and compositions that are suitable for photolithography processes employing KrF, ArF, EUV and similar light sources.
2. Description of the Prior Art
Recently, chemical amplification-type DUV photoresists have been investigated in order to achieve high sensitivity in minute image formation processes for preparing semiconductor devices. Such photoresists are prepared by blending a photoacid generator and matrix resin polymer having an acid labile group.
In a photolithography process, an exposure of photoresist to light of a particular wavelength generates an acid from the photoacid generator that is present in the photoresist. This acid causes the main chain or the branched chain of the resin to decompose or become cross-linked. In addition, the acid removes the acid labile group and changes the polarity of the photoresist in the exposed region. This polarity change creates a solubility difference between the exposed portion and the unexposed portion in a developing solution, thereby allowing a pattern formation. The resolution of the pattern that is formed depends on the wavelength of the light sourcexe2x80x94i.e., in general, a shorter wavelength allows formation of more minute patterns.
In general, a useful photoresist (hereinafter, abbreviated as xe2x80x9cPRxe2x80x9d) has a variety of desired characteristics, such as excellent etching resistance, heat resistance and adhesiveness. In addition, a photoresist should be easily developable in a commercially readily available developing solution, such as 2.38% aqueous tetramethylammonium hydroxide (TMAH) solution. However, it is very difficult to synthesize a photoresist polymer that satisfies all of these requisites. For example, a polymer having a polyacrylate polymer backbone are readily available, but it has poor etching resistance and is difficult to develop. In order to increase its etching resistance, several groups have added an alicyclic unit to the polymer backbone. However, a photoresist copolymer comprising entirely of alicyclic polymer backbone is difficult to form.
To solve some of the problems described above, Bell Research Center developed a polymer having the following chemical structure, where the polymer backbone is substituted with norbornene, acrylate and maleic anhydride units. 
However, the above polymer has a problem in that even in the unexposed regions the maleic anhydride moiety (xe2x80x98Axe2x80x99 portion), which is employed to polymerize alicyclic olefin groups, dissolves readily in 2.38 wt % aqueous TMAH solution. One solution to this problem is to increase the ratio of xe2x80x98Yxe2x80x99 portion having a tert-butyl substituent, but this increase results in a relative decrease in the amount of xe2x80x98Zxe2x80x99 portion (which is responsible for the adhesiveness of the photoresist polymer to the substrate). This decrease in the relative amount of the xe2x80x98Zxe2x80x99 portion may result in separation of the photoresist from the substrate during a pattern formation.
In order to solve the problem, cholesterol-type dissolution inhibitors have been added to the polymer to form a two-component system. However, since the amount of the dissolution inhibitor is very high [about 30% (w/w) of the resin], reproducibility of a pattern is low and the production cost is high, thereby making the system unsuitable as a PR.
In addition, many attempts have been made to improve the etching resistance of photoresist resins (see, for example, Journal of Photopolymer Science and Technology, Vol. 10, No. 3, 511-520 (1997)). Typically, the thickness of a photoresist layer must be less than 0.3 xcexcm in order to form an ultra-fine pattern less than 0.10 xcexcm. However, in most cases, a photoresist layer less than 0.3 xcexcm is too thin to endure the etching gas used in the semiconductor device preparation process.
An object of the present invention is to provide novel photoresist polymers having excellent etching resistance, adhesiveness and photosensitivity, and a process for preparing the same.
Another object of the present invention is to provide photoresist compositions comprising the PR polymers described above, and a process for preparing the same.
Still another object of the present invention is to provide a semiconductor element produced by using the photoresist composition.